Abstract

Time series data of temperature and concentrations of dissolved oxygen and nutrients (total nitrogen – TN, total phosphorus – TP, nitrate – NO³, ammonium – NH₄ and filterable reactive phosphorus - FRP) were examined for trends in Lake Rotoiti for 1981-82 and 1990-2003 and in Lake Rotorua for 2002-03. TP, NO₃, NH₄ in bottom waters (40-60m) of Lake Rotoiti, below the thermocline, were examined using regression equations to quantify seasonal transformations of TP, NO₃ and NH₄. There is a relatively linear build-up of phosphorus through time in the hypolimnion following the onset of stratification and initiation of deoxygenation until the time of winter mixing. When there were high frequency nutrient data for Lake Rotoiti (1981-82 at 60 m, 1991-94 at 60 m and 2001-02 at 40 m depth) the rate of seasonal phosphorus build-up could be quantified by regression to within a relatively narrow range of 111 mg TP m⁻³ yr⁻¹ (0.304 mg TP m⁻³ day⁻¹ ) in 1992-93 to 136 mg TP m⁻³ yr⁻¹(0.373 mg TP m⁻³ day⁻¹ )in 1981-82. Based on the first and last points of all the regression equations developed for TP increase, approximately 14.4 (1993-94) to 20.5 tonnes (2001-02) of phosphorus may be added to the hypolimnion of Lake Rotoiti during stratification. There is no evidence of a consistant interannual change in the seasonal build-up of phosphorus over the years considered and the reasons for interannual variations in the rates are not immediately apparent.